Uranium pentafluoride explained

Uranium pentafluoride is the inorganic compound with the chemical formula UF₅. It is a pale yellow paramagnetic solid. The compound has attracted interest because it is related to uranium hexafluoride, which is widely used to produce uranium fuel. It crystallizes in two polymorphs, called α- and β-UF₅.

Synthesis and structure

Uranium pentafluoride is an intermediate in the conversion of uranium tetrafluoride to volatile UF₆:

2UF₄ + F₂ → 2UF₅

2UF₅ + F₂ → 2UF₆

It can be produced by reduction of the hexafluoride with carbon monoxide at elevated temperatures.^[1]

2UF₆ + CO → 2UF₅ + COF₂Other reducing agents have been examined.^[2]

The α form is a linear coordination polymer consisting of chains of octahedral uranium centers in which one of the five fluoride anion forms a bridge to the next uranium atom.^[3] The structure is reminiscent of that for vanadium pentafluoride.

In the β form, the uranium centers adopt a square antiprismatic structure.^[4] The β polymorph gradually converts to α at 130 °C.^[3]

Monomeric UF₅

Of theoretical interest, molecular UF₅ can be generated as a transient monomer by UV-photolysis of uranium hexafluoride. It is thought to adopt a square pyramidal geometry.^[5]

Notes and References

1. Gordon W. Halstead, P. Gary Eller "Uranium(V) Fluorides and Alkoxides" Inorganic Syntheses 1982, volume 21, 162.
2. Inorganica Chimica Acta . 1979 . 37 . 2 . 129–133 . Crystal Structures of α-UF₅ and U₂F₉ and Spectral Characterization of U₂F₉ . Eller, P. G.. Larson, A. C.. Peterson, J. R.. Ensor, D. D.. Young, J. P.. 10.1016/S0020-1693(00)95530-0 .
3. . 1982 . 45 . 3 . 396–398 . Crystallographic Parameters in α-UF₅ and U₂F₉ by Multiphase Refinement of High-Resolution Neutron Powder Data . Howard, C. J.. Taylor, J. C.. Waugh, A. B.. 10.1016/0022-4596(82)90185-2. 1982JSSCh..45..396H .
4. . 1980 . 35 . 2 . 137–147 . Neutron Diffraction Study of β-Uranium Pentafluoride Between 77 and 403 K. Taylor, J. C.. Waugh, A. B.. 10.1016/0022-4596(80)90485-5 . 1980JSSCh..35..137T .
5. Structure and Bond Nature of the UF₅ Monomer . Onoe, J. . Nakamatsu, H. . Mukoyama, T. . Sekine, R. . Adachi, H. . Takeuchi, K. . Inorg. Chem. . 1997 . 36 . 9 . 1934–1938 . 10.1021/ic961237s . 11669800 .